linux-kernel - Re: [PATCH] dev: use name hash for dev_seq

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Date:	Mon, 10 Oct 2011 11:43:20 +0300
From:	Mihai Maruseac <mihai.maruseac@...il.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	davem@...emloft.net, eric.dumazet@...il.com,
	mirq-linux@...e.qmqm.pl, therbert@...gle.com, jpirko@...hat.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	dbaluta@...acom.com, Mihai Maruseac <mmaruseac@...acom.com>
Subject: Re: [PATCH] dev: use name hash for dev_seq_ops.

On Fri, Oct 7, 2011 at 7:24 PM, Stephen Hemminger <shemminger@...tta.com> wrote:
> On Fri,  7 Oct 2011 18:20:49 +0300
> Mihai Maruseac <mihai.maruseac@...il.com> wrote:
>
>> Instead of using the dev->next chain and trying to resync at each call to
>> dev_seq_start, use this hash and store bucket number and bucket offset in
>> seq->private field.
>>
>> As one can notice the improvement is of 1 order of magnitude.
>
> Good idea,
> This will change the ordering of entries in /proc which may upset
> some program, not a critical flaw but worth noting.
>
> Rather than recording the bucket and offset of last entry, another
> alternative would be to just record the ifindex.
>

I tried to record the ifindex but I think that using it and
dev_get_by_index can result in an infinite loop or a NULL
dereferrence. If a device is removed and ifindex points to it we'll
get a NULL from dev_get_by_index. Checking for NULL and calling again
dev_get_by_index will end in an infinite loop at the end of the hlist.

Augmenting the structure to also contain the number of indexes when
the seq_file is opened returns to the current situation with two ints.
Also, it is more prone to bugs caused by device removal while
printing.

-- 
Mihai
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